<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    .flex-wrapper {
      display: flex;
      flex-direction: column;
      height: 100vh;
    }

    header {
      height: 50px;
    }

    .content {
      flex: 1;
    }

    footer {
      height: 50px;
    }
  </style>
</head>
<body>
  <div class="flex-wrapper">
    <header></header>
    <mm class="content">
      <div>111</div>
    </mm>
    <footer></footer>
  </div>
</body>
</html>